1. Field of the Invention
The present invention relates to a fixing device for fixing an image by heating a printing medium, and an image forming apparatus, such as a copier, a printing, a fax, a multifunctional machine, etc., employing the fixing device, and a control method of controlling the fixing device.
2. Discussion of the Background Art
In the conventional image forming apparatus, there is widely provided a fixing device for fixing an image by heating a toner image transferred onto a printing sheet. The fixing device generally includes a rotational fixing member, a heat generating member for heating the fixing member, and a pressurizing member for applying pressure to the fixing member. Temperature of the fixing member is appropriately controlled to maintain an optimum target level, so that the toner image on the printing sheet can be fixed when the printing sheet passes through a pressure contact section in which the fixing member and the pressurizing member pressure contact each other.
In the above-mentioned fixing device, when a problem, such as short-circuiting, etc., occurs in a control element (e.g. a triac) controlling a heater serving as a heat generating member, heat application by means of the heater becomes uncontrollable, and temperature of a prescribed member heated by the heater sometimes extraordinarily increases. In such a situation, the fixing device is possibly damaged. Then, the fixing device employs a device for detecting such an extraordinary high temperature to forcibly turn off power supply to the heater when detecting thereof.
However, when temperature rise is erroneously detected as being extraordinarily high (i.e. high temperature abnormality) in a normal operation, usability is spoiled. Thus, high temperature abnormality is desirously reliably detected during the normal operation.
For example, as discussed in the Japanese Patent Application Laid Open No. 11-191481, when prescribed high temperature is detected, power is not immediately stopped supplying to the heater, and is stopped only when the high temperature is exceeded and maintained for a prescribed time period thereafter. Thus, even if the prescribed high temperature is temporarily reached by over shooting during the normal operation, it is not regarded as high temperature abnormality.
Further, a fixing device described in the Japanese Patent Application Laid Open No. 2004-219871 has two steps of detecting prescribed high temperature. Specifically, when a condition in that a prescribed lower side high temperature is exceeded and kept for more than a prescribed time period, alarm is initially generated. When a prescribed higher side high temperature is detected, power is not immediately stopped supplying to a heater, and is stopped only when the prescribed higher side high temperature is exceeded and maintained for more than a prescribed time period thereafter. Thus, similar to the fixing device of the Japanese Patent Application Laid Open No. 11-191481, the Japanese Patent Application Laid Open No. 2004-219871 prevents temperature rise from being regarded as high temperature abnormality in the ordinary operation.
However, the Japanese Patent Application Laid Open Nos. 11-191481 and 2004-219871 are possibly incapable of detecting the high temperature abnormality when a thermistor or the like arranged to detect the same is rarely malfunctioned due to an accident, such as imperfect short circuitry, etc.